Method for producing carbon nanotubes and/or nanofibres

ABSTRACT

A method for producing aligned carbon nanotubes and/or nanofibres comprises providing finely divided substrate particle having substantially smooth faces with radii of curvature of more than 1 μm and of length and breadth between 1 μm and 5 mm and having catalyst material on their surface and a carbon-containing gas at a temperature and pressure at which the carbon-containing gas will react to form carbon when in the presence of the supported catalyst, and forming aligned nanotubes and/or nanofibres by the carbon-forming reaction.

This application is the national stage of International (PCT) PatentApplication Ser. No. PCT/GB2003/004925, filed Nov. 13, 2003, whichclaims priority to British Application No. 0226590.8, filed Nov. 14,2002, the disclosure of each of which is incorporated herein byreference.

The present invention relates to a method of producing carbon nanotubesand/or nanofibres, and to nanotubes and/or nanofibres so produced.

Carbon nanotubes are tubular fullerenic structures which may besingle-walled or multi-walled. Carbon nanofibres are fullerenicnanostructures in which the graphitic layers are not tubular: forexample, solid fullerenic cylinders or hollow cylindrical fullerenicstructures wherein the fullerene net is not parallel to the cylinderaxis.

Carbon nanotubes and nanofibres have remarkable mechanical andelectrical properties and are being investigated for many potentialapplications. These materials have been produced previously by usingvarious approaches, including laser or arc-discharge ablation of acarbon/catalyst mixture target. The materials are usually produced in anentangled state.

For larger scale synthesis, the most promising methods have been basedon chemical vapour decomposition (CVD). In these methods, a carboncontaining gas is decomposed at high temperature under the influence ofa finely divided transition metal catalyst.

Large quantities of multi-walled nanotubes can be grown using CVD bymethods well known in the literature (H. G. Tennant, U.S. Pat. No.5,165,909). However, this material, sometimes termed ‘cotton candy’, ishighly entangled, a characteristic that reduces its utility in certainapplications (for example, this material is not readily dispersible as afiller). In addition, nanotubes grown in this way do not have a clearlydefined length and often have to be broken up into shorter sectionsusing aggressive chemical treatments (e.g. boiling in concentratednitric acid) in order to achieve a good dispersion (Shaffer).Particulate substrates have also been used as catalyst supports forgrowing entangled nanotubes (e.g. WO00/17102).

WO00/73205 discloses a method for producing carbon nanotubes from carbonmonoxide gas using silica-supported cobalt and molybdenum as a catalyst.The single-walled nanotubes produced are locally roughly aligned inbundles but each nanotube travels between different bundles and thematerial is thus entangled.

Multi-walled nanotubes have also been grown on non-particulatesubstrates (usually quartz plates or furnace tubes with diametersbetween 15 mm and 100 mm) in order to form aligned arrays, also known as‘carpets’, ‘forests’, or ‘grass’ (e.g. Andrews). The process usuallyinvolves the thermal decomposition of ferrocene/toluene mixtures at600-1000° C. although a variety of related feedstocks may be used. Byvarying processing parameters such as time, temperature, and catalystconcentration it is possible to adjust the length, diameter, and packingdensity of the nanotubes formed within certain ranges (e.g. Singh).Aligned nanotubes have also widely been prepared on flat substrates byusing various types of plasma enhancement (for example microwave,radio-frequency or direct current) (e.g. Ren). Aligned nanotubes grownby thermal CVD often have a higher crystalline quality than their‘cotton candy’ counterparts.

The present invention provides in a first aspect a method for producingaligned carbon nanotubes and/or nanofibres comprising providing finelydivided substrate particles having substantially smooth faces with radiiof curvature of more than 1 μm and of length and breadth between 1 μmand 5 mm having a catalyst material on their surface and acarbon-containing gas at a temperature and pressure at which thecarbon-containing gas will react to form carbon when in the presence ofthe supported catalyst, and forming aligned nanotubes and/or nanofibresby the carbon-forming reaction.

At the nanometer scale (less than 100 nm), the surface may have atexture or roughness that stabilises metal catalyst clusters of suitablesizes for nanotube and/or nanofibre growth. The surface should be smoothover the order of size of catalyst material clusters.

Preferably, the faces have radii of curvature of more than 10 μm. Morepreferably, the substantially smooth faces are substantially flat.

Preferably, the catalyst material is dispersed in clusters on thesurface of the substrate particles. More preferably, the catalystmaterial clusters are from 0.5 nm to 100 nm in dimension. Highlypreferably, the catalyst material clusters are from 3 nm to 50 nm indimension.

Optionally, the substrate particles having catalyst material on theirsurface are prepared by depositing catalyst material on the surface ofthe substrate particles.

Metal or metal salts may be introduced onto the substrate particles by arange of treatments including electroless deposition, solvent drying,supercritical drying, sputtering, physical vapour deposition, orelectroplating. Subsequent thermal and/or oxidising or reducing gastreatments may be used in converting a given salt to a metal or to asuitable precursor for thermal decomposition (see below). Continuousfilms may be broken up on thermal treatment to form suitable catalystparticles by a dewetting process.

Alternatively, the substrate particles having catalyst material on theirsurface may be prepared by providing finely divided substrate particlesand a catalyst precursor material and decomposing the catalyst precursormaterial to form the catalyst material in the presence of the substrateparticles such that the catalyst material is deposited on the substrateparticles. In a preferred embodiment of the invention, the catalystprecursor material is decomposed while the substrate particles are incontact with the carbon-containing gas, for example within the main CVDfurnace.

As a further alternative, the substrate particles having catalystmaterial on their surface may be prepared by providing finely dividedsubstrate particles of a material that is decomposable to form catalystmaterial on the surface of non-catalyst substrate particles anddecomposing said material. Some perovskites are known to behave in thisway (e.g. Liang).

Preferably, the length and breadth of the substrate particle faces arebetween 10 μm and 500 μm. If the particle is facetted, the majority offacets are preferably more than 25 μm² in area and most preferably widerthan 2 μm in each direction.

The substrate may be, by way of example, ceramic (silica or alumina),mineral (mica), metallic (titanium), salt (sodium chloride, magnesiumoxide, calcium oxide) or carbon-based. The substrate particles may be ofgraphite, aluminium or titanium. Preferably, the substrate particles areof silica, carbon, magnesium oxide, calcium oxide, or sodium chloride.In the most straightforward case, the substrate particles are simplyfinely ground powders, such as silica or alumina, or drawnaluminosilicate fibres, or processed minerals such as mica. Othermaterials may be generated by a range of methods, known to those skilledin the art, such as colloidal processing, spray-drying, hydrothermalprocessing, ball-milling extrusion and so on. Freshly prepared materialswith uncontaminated surfaces (e.g. as obtained by prompt use of ballmilled silica, milled using silica-based balls) are preferably used inorder to obtain best results. Preferably, the materials are used withina day of preparation.

Preferably, the substrate particles are anisotropic in order to providea large surface area to volume ratio. More preferably, the substrateparticles have one dimension larger than the other two dimensions or thesubstrate particles have one dimension smaller than the other twodimensions.

Optionally, the substrates are coated with a buffer layer. The bufferlayer on the surface of the substrate particles serves either to supportor enhance the catalyst particles or to isolate the growth process fromthe underlying material. As an example, such an approach could be usefulwhen using a substrate that is convenient to remove after the growthreaction but which has a relatively high solubility for the catalystmetal (such as a magnesium oxide/nickel system). Such a buffer layer maybe introduced by deliberate treatment (Sun) or may arise naturally (e.g.the oxide layer on titanium particles).

Preferably, the catalyst material is a transition metal, an alloy of twoor more thereof, a compound of a transition metal or a mixture of two ormore compounds of transition metals. Particularly suitable are the GroupVIB chromium (Cr), molybdenum (Mo), tungsten (W) or Group VIIIBtransition metals, e.g., iron (Fe), cobalt (Co), nickel (Ni), ruthenium(Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) andplatinum (Pt) or mixtures thereof. Metals from the lanthanide andactinide series may also be used. More preferably, the transition metalis iron, cobalt, molybdenum or nickel or mixtures thereof.

Preferably, the catalyst precursor is a transition metal carbonyl, or atransition metal cyclopentadienyl compound. More preferably, thecatalyst precursor is ferrocene, nickelocene, cobaltocene, ironpentacarbonyl or nickel tetracarbonyl. The catalyst precursor compoundmay provide the carbon-containing gas, for example where ferrocene isused to provide iron catalyst and cyclopentadiene carbon-containing gas.

Preferably, the carbon containing gas is carbon monoxide, an oxygencontaining organic compound or a hydrocarbon, or a mixture of two ormore thereof. More preferably, the carbon containing gas is carbonmonoxide, benzene, toluene, xylene, cumene, ethylbenzene, naphthalene,phenanthrene, anthracene, methane, ethane, propane, hexane, ethylene,propylene, acetylene, formaldehyde, acetaldehyde, acetone, methanol,ethanol or a mixture of two or more thereof. In preferred embodiments,the carbon-containing compound is toluene, xylene or benzene. It is ofcourse only required that the reactant be gaseous under the reactionconditions.

Optionally, one or more boron and/or nitrogen containing compound isprovided in addition to the carbon containing gas. It has been shown inflat substrate studies that boron or nitrogen can be introduced into thecarbon lattice (Terrones), by using boron or nitrogen containing speciesin the feedstock for the CVD reaction. Accordingly, it is possible togrow large volumes of such ‘doped’ materials using this invention by theaddition of suitable species, such as ammonia, pyridine, aniline,borazine, borane, phthalocyanines, to the feedstock.

Optionally, one or more promoter compounds is provided in addition tothe carbon containing gas. Promoters aid the catalytic selectivity orreactivity. These agents may be added as the catalyst is formed orduring growth of the nanotubes and/or nanofibres. For example, thiophenemay be added to encourage the production of herringbone fibres orsingle-walled nanotubes (Singh, Zhu).

Optionally, a diluent gas is provided mixed with the carbon containinggas. The diluent is preferably an inert gas, e.g. argon. Thecarbon-containing gas may also be mixed with non carbon-containing gasesthat play no direct role in the carbon-forming reaction but which play acontributory role, for instance by reacting with amorphous carbon as itis formed (as a by-product) and so keeping the reaction sites on thecatalyst clean and available for nanotube formation. Gases which may bemixed with the carbon-containing gas include argon, hydrogen, nitrogen,ammonia, or helium.

The substrate particles may be introduced into the CVD reactorbatchwise, or added to the furnace continuously either using standardpowder handling techniques or by carrying in a suitable gas (seePCT/GB02/02239). Preferably, substrate particles are provided andproduct particles are removed from a reaction vessel in a continuousfashion. Once introduced to the furnace, the substrate particles may sitin a fixed bed, a fluidised bed, or be carried within the gas flows inthe system.

Preferably, the method described above further comprises the step ofrecovering the aligned nanotubes and/or nanofibres.

Preferably, gaseous effluent from the reaction is recycled with orwithout clean up.

Suitably, formation of the nanotubes and/or nanofibres takes place at atemperature of from 650° C. to 1250° C., e.g. 650° C. to 850° C.Preferred gas pressures are from 0.1 to 50 bar A, preferably from 0.5 to5 bar A, more preferably 1 to 2 bar A. The ratio of catalyst metal tocarbon fed to the reaction zone is preferably less than 1:100, e.g.1:100 to 1:500.

In a second aspect, the invention relates to finely divided substrateparticles having substantially smooth faces with radii of curvature ofmore than 1 μm and of length and breadth between 1 μm and 5 mm with acatalyst material on the surface of the substrate particles and withaligned carbon nanotubes and/or nanofibres on the surface of thesubstrate particles. The nanotube and/or nanofibre coated substrateparticles may be used directly (with either or both componentscontributing to a desired active or passive functionality).

Alternatively, the nanotubes or nanofibres may be removed from thesubstrate by dissolution of the substrate (e.g. hydrogen fluoridetreatment of silica, or hydrothermal treatment of sodium chloride) or bydissolution of the catalyst or buffer layers (e.g. iron in dilutehydrochloric acid). The now disconnected nanotubes and/or nanofibres andsubstrates may be conveniently separated by filtration or sedimentation.In particular the disconnected materials may be placed in a medium ofintermediate density such that the substrate particles sink and thenanotubes and/or nanofibres rise (or vice versa). After separation, thesubstrate particles may be reused.

In a third aspect, the invention relates to nanotubes and/or nanofibresproduced by a method as described above. Preferably, the nanotubesand/or nanofibres are separated from the substrate particles by partialor complete dissolution of the substrate particles or catalystmaterials.

The invention will be further described with reference to the followingExample and with reference to the Figures, in which:

FIG. 1 shows an SEM image of aligned nanotubes grown on quartz substrateat −700° C. for 90 minutes.

FIG. 2 shows a Raman spectrum taken from the nanotubes grown at 700° C.taken using a 514 nm excitation laser.

FIG. 3 shows TGA on nanotubes grown at 700° C. for 90 minutes, 750 mlmin⁻¹ Ar:H₂ flow rate and at a feed rate 1.2 ml hr⁻¹ of the solution.

EXAMPLE

A solution containing 2 wt % of ferrocene dissolved into toluene wasprepared. 100 mg of high surface area quartz powder was prepared by ballmilling. Milling time was 2 hours using alumina ball-bearings. Thesilica broke up into flakes approximately 10 to 40 μm in diameter andaround 1 μm thick. The ground silica was sorted with a 75 μm sieve. Thefine powder was then placed into a horizontal tube furnace (internaldiameter 14 mm, length 90 cm) and heated to 700° C. The solution wassprayed into the furnace at 1.2 ml hr⁻¹ using a dry argon-hydrogenatmosphere for 90 minutes. The ratio of argon to hydrogen was 14:1 witha total gas flow rate of 750 ml min⁻¹. The products were characterisedby electron microscopy, both scanning (SEM) and high-resolutiontransmission electron microscopy (HRTEM), and Raman spectroscopy andThermogravimetric analysis (TGA). From FIG. 1 it can be seen that thenanotubes are in abundance, high purity, and aligned with constantlengths. The nanotubes are mainly in the bundled form growingperpendicular to the surface of the support. The average diameters ofthe nanotubes were 27±2.7 nm determined using TEM.

In FIG. 2 the quality of the material based upon the relative intensityof the peak at ˜1350 cm⁻¹ to ˜1580 cm⁻¹ indicates that the nanotubescontain few defects. TGA (FIG. 3) indicates that approximately 40% ofthe weight loss was due to the nanotubes. The remaining powder was ironoxide supported on quartz. The average weight loss from TGA was ˜32.5%.

The advantages of the technique of the Example for large scaleproduction of carbon nanotubes are summarised as follows:

-   1. Low production cost: the raw materials involved for forming the    support, catalyst and carbon feedstock are cheap and readily    available in large quantities.-   2. The method results in high volumes and high yield of good quality    pure nanotubes, and can readily be scaled up to an industrial level.-   3. Characteristics such as nanotube/nanofibre diameter and length    can be manipulated by the growth process.

The disadvantage of known flat substrate methods of production comparedwith the method of the Example is that the ratio between the growingsurface area and volume of support is low as a result of the geometry ofthe plates. Because the growth is confined to a macroscopic surface thetotal volumetric yield of product is low.

The disadvantage of the method of WO00/73205 compared with the method ofthe Example is that aligned, non-entangled nanotubes are not produced.This is apparently because the substrate particles used in that methoddo not have the required characteristics.

Whilst the applicants do not wish to be bound by this theory, it isbelieved that during growth the aligned nanotube or nanofibre arraysgrow from all sides of the substrate particle, leading to a large volumeof carbon product. During growth the substrate particles move apart inorder to accommodate the growing nanotubes.

REFERENCES

-   Andrews, R., Jacques, D., Rao, A. M., Derbyshire, F., Qian, D., Fan,    X., Dickey, E. C., and Chen, J., Continuous production of aligned    carbon nanotubes: a step closer to commercial realization. Chemical    Physics Letters, 1999. 303(5-6): p. 467-474.-   Ren, Z. F., Huang, Z. P., Xu, J. W., Wang, J. H., Bush, P.,    Siegal, M. P., and Provencio, P. N., Synthesis of large arrays of    well-aligned carbon nanotubes on glass. Science, 1998. 282(5391): p.    1105-1107.-   Liang, Q., Tang, S. H., Gao, L. Z., Chen, Z. Y., Zhang, B. L.,    Yu, Z. L., A study on production of carbon nanotubes by    decomposition of CH ₄ over the pre-reduced catalysts LaNiO ₃ , La ₄    Ni ₃ O ₁₀ , La ₃ Ni ₂ O ₇ and La ₂ NiO ₄, Acta Chim. Sin., 2001, 59,    8, p. 1236-1240.-   Sun, X., Stansfield, B., Dodelet, J. P., Desilets, S., Growth of    carbon nanotubes on carbon paper by ohmically heating    silane-dispersed catalytic sites, Chem. Phys. Lett. 363 (2002)    415-421.-   Production of aligned carbon nanotubes by the CVD injection method,    Singh C, Shaffer M, Kinloch I, and Windle A, Physica B, Vol 323, No.    1-4, 339-340, 2002-   Shaffer M S P, Fan X, Windle A H, Dispersion and packing of carbon    nanotubes, Carbon, 1998, Vol. 36, No. 11, 1603-1612-   Terrones, M., Benito, A. M., MantecaDiego, C., Hsu, W. K., Osman, O.    I., Hare, J. P., Reid, D. G., Terrones, H., Cheetham, A. K.,    Prassides, K., Kroto, H. W., and Walton, D. R. M., Pyrolytically    grown BxCyNz nanomaterials: Nanofibres and nanotubes. Chemical    Physics Letters, 1996. 257(5-6): p. 576-582.-   Terrones, M., Redlich, P., Grobert, N., Trasobares, S., Hsu, W. K.,    Terrones, H., Zhu, Y. Q., Hare, J. P., Reeves, C. L., Cheetham, A.    K., Ruhle, M., Kroto, H. W., and Walton, D. R. M., Carbon nitride    nanocomposites: Formation of aligned CxNy nanofibers. Advanced    Materials, 1999. 11(8): p. 655-   Singh, C., Quested, T., Boothroyd, C., Thomas, P., Kinlock, I., Abou    Kandil, A., Windle, A. H., Synthesis and characterisation of carbon    nanofibres produced by the floating catalyst method, J. Phys. Chem.    B., 2002, 106, p. 10915-10922.-   Zhu, H. W., Xu, C. L., Wu D. H., Wei B. Q., Vajtai R., Ajayan P. M.,    Direct synthesis of single-walled carbon nanotube strands, Science,    2002, 296, 884-886

1. A method for producing aligned carbon nanostructures comprising: (a)providing (i) finely divided substrate particles having substantiallysmooth faces with radii of curvature of more than 1 μm and of length andbreadth between 1 μm and 5 mm and having a catalyst material on theirsurface and (ii) a carbon-containing gas at a temperature and pressureat which the carbon-containing gas will react to form carbon when in thepresence of the catalyst material, and (b) forming alignednanostructures by the carbon-forming reaction, wherein the substrateparticles are of silica, alumina, carbon, mica, magnesium oxide, calciumoxide, sodium chloride, or a mixture of two or more thereof, or are ofgraphite, aluminium, or titanium, and further wherein the substrateparticles are freshly prepared by colloidal processing, spray-drying,hydrothermal processing, or ball milling, and further wherein thesubstrate particles having the catalyst material on their surface areprepared by depositing the catalyst material on the surface of thesubstrate particles by electroless deposition, solvent drying,supercritical drying, sputtering, physical vapour deposition orelectroplating, and further wherein the catalyst material is atransition metal, an alloy of two or more thereof, a compound of atransition metal or a mixture of two or more compounds of transitionmetals, where the transition metal is iron, cobalt, molybdenum ornickel.
 2. A method as claimed in claim 1, wherein the faces have radiiof curvature of more than 10 μm.
 3. A method as claimed in claim 1,wherein the substantially smooth faces are substantially flat.
 4. Amethod as claimed in claim 1, wherein the catalyst material is dispersedin clusters on the surface of the substrate particles.
 5. A method asclaimed in claim 4, wherein the catalyst material clusters are from 0.5nm to 100 nm in dimension.
 6. A method as claimed in claim 5, whereinthe catalyst material clusters are from 3 nm to 50 nm in dimension.
 7. Amethod as claimed in claim 1, wherein the length and breadth of thesubstrate particle faces are between 10 μm and 500 μm.
 8. A method asclaimed in claim 1, wherein the substrate particles are anisotropic. 9.A method as claimed in claim 8, wherein the substrate particles have onedimension larger than the other two dimensions or wherein the substrateparticles have one dimension smaller than the other two dimensions. 10.A method as claimed in claim 1, where the substrates are coated with abuffer layer.
 11. A method as claimed in claim 1, wherein the catalystmaterial is ferrocene, nickelocene, cobaltocene, iron pentacarbonyl, ornickel pentacarbonyl.
 12. A method as claimed in claim 1, wherein thecarbon containing gas is carbon monoxide, an oxygen containing organiccompound or a hydrocarbon, or a mixture of two or more thereof.
 13. Amethod as claimed in claim 12, wherein the carbon containing gas iscarbon monoxide, benzene, toluene, xylene, cumene, ethylbenzene,naphthalene, phenanthrene, anthracene, methane, ethane, propane, hexane,ethylene, propylene, acetylene, formaldehyde, acetaldehyde, acetone,methanol, ethanol or a mixture of two or more thereof.
 14. A method asclaimed in claim 1, wherein one or more boron and/or nitrogen containingcompound is provided in addition to the carbon containing gas.
 15. Amethod as claimed in claim 1, wherein one or more promoter compounds isprovided in addition to the carbon containing gas.
 16. A method asclaimed in claim 15, wherein the promoter compound is thiophene.
 17. Amethod as claimed in claim 1, wherein a diluent gas is provided mixedwith the carbon containing gas.
 18. A method as claimed in claim 1,wherein the substrate particles are reacted within a fluidised bed. 19.A method as claimed in claim 1, wherein substrate particles are providedand product particles are removed from a reaction vessel in a continuousfashion.
 20. A method as claimed in claim 1, further comprising the stepof recovering the aligned nanostructures.
 21. A method as claimed inclaim 1, wherein gaseous effluent from the reaction is recycled with orwithout clean up.
 22. A method as claimed in claim 1, wherein thereaction takes place at a temperature between 650° C. and 1250° C. 23.Carbon nanostructures produced by the method as claimed in claim
 1. 24.Carbon nanostructures as claimed in claim 23, wherein the nanostructuresare separated from the substrate particles by partial or completedissolution of the substrate particles or catalyst materials.